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In battling with other players(wi-fi,PO, etc), some Pokemon like Legendary Pokemon were more powerful then other Pokemon, with many having over 100 Base Points in HP, Attack, Defense, Special Attack, Special Defense and Speed; This caused many problems, until tiers were introduced.
There are many tiers, like OU, and NU. These tiers allow only some Pokemon, and the stronger ones are banned from being used to it. Here is a list of tiers :

Uber ( Very strong Pokemon like Legendaries are allowed to battle in this tier; No Pokemon are banned here)

OU (Slightly less stronger Pokemon are used, like Infernape, which are highly used.Ubers are banned here.OU stands for Over Used)

UU (weaker Pokemon reside here, and OU and Uber Pokemon are banned from usage. UU is short for Underused)